1. Field of the Invention
This invention relates to electrical terminal pins having a compliant portion adapted to be inserted into a plated-through hole in a circuit board or the like.
2. Brief Description of the Prior Art
Terminal pins with compliant sections or portions (sometimes called press-fit pins) have been known in the art for over thirty years. Compliant pins are design to be inserted into a plated-through hole in a printed circuit board. The pin generally includes a mating portion adapted to contact an electrically conductive element and a compliant portion extending from the mating portion and adapted to make electrical contact with conductive material defining the interior surface of the plated-through hole.
Generally speaking, the following characteristics are desirable in a compliant pin:
1. Soldering is unnecessary for high reliability applications.
2. The pins should be cyclable, i.e., the pins should be able to withstand repeated insertions and withdrawals from the plated-through hole. This allows any defective connection with the board to be easily repaired.
3. If there is any damage during the insertion, it should only occur to the pin and not the printed circuit board or the conductive material lining the hole.
4. Elastic strain energy should be largely stored in the compliant portion of the pin.
5. Pins should be able to be used over a wide range of hole sizes. This would eliminate the need for different thicknesses of the plating material formed in the hole.
6. Relatively lower insertion forces should be provided so that mass insertion is feasible.
7. If there is a permanent set as between the compliant portion and the plated-through hole, the smaller set should occur to the hole. This would allow for lower local stresses and thinner printed circuit boards.
8. The insertion force of the pin should be as nearly equal to the push out or retention force as possible.
9. The largest possible area of the compliant portion should engage the interior of the plated-through hole with the largest possible normal force.
10. Once fully inserted into a plated-through hole, the top or mating portion of the pin should be resistant to breakage when it is bent or twisted.
11. The pin should be easily manufactured, preferrably using a flat blank with the same general material thickness.
The various compliant pin designs now on the market are effective to accomplish one or more of the stated objectives listed above. However, as in many design alternatives, the increase in performance with respect to one feature may often result in a decrease in performance with respect to another feature.
It has been found that the cross-section of the compliant portion which offers the best of all of the above features is a generally S-shaped cross-section. Examples of pins or terminals of this type are disclosed in U.S. Pat. No. 3,907,400, U.S. Pat. No. 4,415,220 and Edward H. Key, Electronic Design, "Development of a New Drawn-Wire Compliant Pin", 20th Annual Connectors & Interconnection Technology Symposium, Philadelphia, Pennsylvania, Oct. 19-21, 1987 (the "Key Article").
U.S. Pat. No. 3,907,400 discloses a compliant type post which is adapted to be inserted through a printed circuit board hole. The use of this post in a plated-through hole is not disclosed. The purpose of the post is to have a wire wrap on one side to connect to another component (e.g., another wire wrap) on the other side of the printed circuit board.
U.S. Pat. No. 4,415,220 discloses an S-shaped compliant portion that gradually decreases in diameter from a fully developed section through the transition section ending with an eliptical cross section (see FIGS. 3-6). The fully developed section is of a constant width. Because of the constant width, insertion may cause plastic deformation affecting the normal force generated against the interior of the plated-through hole.
The Key Article also discloses an S-shaped compliant protion whose fully developed section is of constant width and which suffers from the same draw back of undue plastic deformation. Also disclosed is a manufacturing process which produces the pin from drawn wire. This is a relatively inefficient means of mass producing pins of this type.
The deficiencies of the prior art devices fall generally into three different categories:
1. Because of the constant width of the compliant portion there is a plastic deformation which occurs during the insertion process. This phenomenon is best described in FIG. 5 on page 4 of Ram Goel, AMP Incorporated, "An Analysis of Press-Fit Technology", Electronic Components' Conference, Atlanta, Georgia, May 11-13, 1981 (the "Goel Article"). In the Goel Article, it is shown that the middle of the compliant portion of most compliant pins are permanently and plastically deformed inwardly during insertion. As a result, the middle of the compliant portion, which should exert the highest normal force against the interior of the plated-through hole, does not generate high enough forces while still maintaining the necessary compliancy.
2. Many applications for a compliant pin require that it be able to withstand a certain amount of bending and/or twisting after inserted into the plated-through hole. Very often bending and/or twisting the mating portion of the pin results in the breakage of the pin immediately above the level of the printed circuit board. None of the prior art references addresses this problem.
3. It is very important that whatever pin design that is used be easily manufacturable. None of the S-shaped compliant pins of the prior art disclose a mass producable design.